Betulinic acid is useful as a therapeutic agent. For example, Pisha, E. et al., (1995) J. M. Nature Medicine, 1, 1046-1051 disclose that betulinic acid has antitumor activity against melanoma, e.g., MEL-1, MEL-2 and MEL-4. In addition, Fujioka, T. et al., J. Nat. Prod., (1994) 57, 243-247 discloses that betulinic acid has anti-HIV activity in H9 lymphocytic cells.
Betulinic acid can be manufactured from betulin, which is present in large quantities in the outer birch bark of numerous species of birch trees. For example, a single paper mill in northern Minnesota generates nearly 30-70 tons of birch bark per day. Approximately 230,000 tons of birch bark are generated per year. Outer bark of Betula verrucosa (European commercial birth tree) contains nearly 25% betulin (Rainer Ekman, 1983, Horzforschung 37, 205-211). The outer bark of Betula paparifera (commercial birch of northern U.S. and Canada) contains nearly 5-18% etulin (see, U.S. patent Ser. No. 09/371298). As such, vast quantities of betulin are available.
U.S. Pat. No. 5,804,575 issued to Pezzuto et al. discloses a five-step process for the synthesis of betulinic acid from betulin. Due to the length of time required to carry out this process and the yield it provides, it is not ideal for the commercial scale (e.g., kilogram) production of betulinic acid. Additionally, the process uses solvents and reagents that are hazardous and expensive, and the disclosed purification steps are not feasible on a commercial scale.
The first step in the preparation of betulinic acid from betulin-3-acetate was described by Ruzichka et al. (Helv. Chim. Acta., 21, 1706-1715 (1938)).
The main obstacle for employing this method is the preparation of starting material (i.e., betulin-3-acetate). The selectivity of the hydrolysis of betulin-3, 28-diacetate with potassium hydroxide provided about 60% betulin-3-acetate. The use of magnesium alcoholates (Yao-Chang Xu et al., J. Org. Chem., 61, 9086-9089 (1996)) in the selective deprotection of betulin-3,28-diacetate (Yao-Chang Xu et al., J. Org. Chem., 61, 9086-9089 (1996)) has several serious drawbacks. The selectivity of this process is about 81%. Additionally, the cost of magnesium alcoholates is fairly high. As such, this method is not attractive for the commercial scale production of betulinic acid.
Thus, there exists a need for improved methods for preparing betulinic acid and synthetic precursors thereof. Such improved methods should require less time, should provide a higher overall yield, should be more cost effective (i.e., should require less expensive reagents and solvents) than known procedures, or should satisfy the contemporary industrial demands from both safety and environmental points of view.